System for controlling the differential pressure of a room having laboratory fume hoods

ABSTRACT

A system whereby the pressure of a room containing one or more fume hoods, such as a laboratory, is to maintained at a predetermined level relative to the pressure of a reference space in the building, which may be the pressure in an adjacent corridor or an adjacent room or the like. The system involves a room controller which is part of the heating, ventilating and air conditioning apparatus of the building. The room controller is of the type which can receive electrical signals from each fume hood controller, which signals are proportional to the volume of air that is being exhausted through each fume hood. The volume indicating signals communicated from each of the fume hood controllers to the room controller enable the system to modulate the volume of air that is being supplied to the room and thereby maintain the differential pressure at the desired level with relatively quick response times.

CROSS REFERENCE TO RELATED APPLICATIONS

1. Title: Apparatus for Controlling the Ventilation of a Laboratory FumeHood

Inventors: Osman Ahmed, Steve Bradley, Steve Fritsche and Steve Jacob

Ser. No.: 590,195

2. Title: Apparatus for Determining the Position of a Moveable StructureAlong a Track

Inventors: David Egbers and Steve Jacob

Ser. No.: 591,102

3. Title: A method and Apparatus for Determining the Uncovered Size ofan Opening Adapted to be Covered by Multiple Moveable Doors

Inventors: Osman Ahmed, Steve Bradley and Steve Fritsche

Ser. No.: 590,194

4. Title: Laboratory Fume Hood Control Apparatus Having Improved SafetyConsiderations

Inventors: Osman Ahmed

Ser. No.: 589,952

The present invention relates generally to the control of theventilation of laboratory fume hoods, and more particularly to a systemfor controlling the differential pressure of a room having one or morelaboratory fume hoods located therein.

Fume hoods are utilized in various laboratory environments for providinga work place where potentially dangerous chemicals are used, with thehoods comprising an enclosure having moveable doors at the front portionthereof which can be opened in various amounts to permit a person togain access to the interior of the enclosure for the purpose ofconducting experiments and the like. The enclosure is typicallyconnected to an exhaust system for removing any noxious fumes so thatthe person will not be exposed to them while performing work in thehood. Fume hood controllers which control the flow of air through theenclosure have grown more sophisticated in recent years, and are nowable to more accurately maintain the desired flow characteristics toefficiently exhaust the fumes from the enclosure as a function of thedesired average face velocity of the opening of the fume hood.

The average face velocity is generally defined as the flow of air intothe fume hood per square foot of open face area of the fume hood, withthe size of the open face area being dependent upon the position of oneor more moveable doors that are provided on the front of the enclosureor fume hood, and in most types of enclosures, the amount of bypassopening that is provided when the sash door or doors are closed.

The fume hoods are typically exhausted by an exhaust system thatgenerally includes a blower that is capable of being driven at variablespeeds to increase or decrease the flow of air from the fume hood tocompensate for the varying size of the opening or face. Alternatively,there may be a single blower connected to the exhaust manifold that isin turn connected to the individual ducts of multiple fume hoods, anddampers may be provided in the individual ducts to control the flow fromthe individual ducts to thereby modulate the flow to maintain thedesired average face velocity.

The doors of such fume hoods can be opened by raising them vertically,often referred to as the sash position, or some fume hoods have a numberof doors that are mounted for sliding movement in typically two sets oftracks. There are even doors that can be moved horizontally andvertically, with the tracks being mounted in a frame assembly that isvertically moveable.

The volume of air that is drawn through each of the fume hoods is atleast partially a function of the uncovered portion of the openingthereof, and if a relatively constant average face velocity ismaintained, then a larger open area of the fume hood will result in moreair being drawn into the fume hood and exhausted from it. Since thetotal number of fume hoods that are present in laboratory rooms can bequite large in many installations, it should be appreciated that asubstantial volume of air may be removed from the laboratory room duringoperation. Also, since the HVAC system supplies air to the laboratoryroom, there may be a substantial change in the volume of air required tobe supplied to a room depending upon whether the fume hoods arefrequently being opened, or other changes occur.

Because much of the work that is performed in many laboratories involveschemicals which may be dangerous, it is often desirable to maintain thedifferential pressure within the laboratory at a lower pressure than thehallways outside of the laboratory or adjacent rooms. If the laboratoryhas several fume hoods which are exhausting air from the room, theamount of air supplied to the laboratory will necessarily be greaterthan a comparably sized room without fume hoods, and there may beincreased difficulty in maintaining the desired differential pressurebetween the laboratory and a reference space if the fume hoods havetheir sash doors frequently opened.

Accordingly, it is one of the primary objects of the present inventionto provide an improved system for controlling the differential pressurebetween a laboratory room which has a number of fume hoods locatedwithin it and the reference space that is preferably adjacent to thelaboratory.

Another object of the present invention is to provide a system whichintegrates a fume hood controller apparatus with the heating,ventilating and air conditioning control equipment for a laboratory roomin which the fume hoods are located, and which achieves greatlyincreased control of the of the HVAC equipment so that the differentialpressure of the room relative to the pressure of a reference space suchas a corridor or an adjacent area can be maintained at a desired level.

Another one of the primary objects of the present invention is toprovide such an improved system for controlling the pressure of alaboratory room which is achieved by providing current and accurateinformation to the HVAC system about the volume of air flow from each ofthe fume hoods.

Stated in more detail, it is an object of the present invention toprovide an improved system for providing control of the differentialpressure of a room having a number of fume hoods with respect to thepressure in a reference space such as a corridor or the like, andwherein the room has a room controller for controlling the volume of airthat is supplied and exhausted from the room, by providing signals fromeach fume hood controller to the room controller which indicates thevolume of air that is being exhausted by each fume hood.

These and other objects will become apparent upon reading the followingdetailed description of the present invention, while referring to theattached drawings, in which:

FIG. 1 is a schematic block diagram of system of the present inventionwhich includes a room controller of a heating, ventilating and airconditioning monitoring and control apparatus of a building, and severalfume hood controllers;

FIG. 2 is a block diagram of a fume hood controller, shown connected toan operator panel, the latter being shown in front elevation;

FIG. 3 is a diagrammatic elevation of the front of a representative fumehood having vertically operable sash doors;

FIG. 4 is a diagrammatic elevation of the front of a representative fumehood having horizontally operable sash doors;

FIG. 5 is a cross section taken generally along the line 5--5 of FIG. 4;

FIG. 6 is a diagrammatic elevation of the front of a representativecombination sash fume hood having horizontally and vertically operablesash doors;

FIG. 7 is an electrical schematic diagram of a plurality of door sashposition indicating switching means;

FIG. 8 is a cross section of the door sash position switching means;

FIG. 9 is a schematic diagram of electrical circuitry for determiningthe position of sash doors of a fume hood;

FIG. 10 is a block diagram illustrating the relative positions of FIGS.10a, 10b, 10c, 10d and 10e to one another, and which together comprise aschematic diagram of the electrical circuitry for a fume hood controllermeans;

FIGS. 10a, 10b, 10c, 10d and 10e, which if connected together, comprisethe schematic diagram of the electrical circuitry for the fume hoodcontroller means;

FIG. 11 is a flow chart of the general operation of the fume hoodcontroller;

FIG. 12 is a flow chart of a portion of the operation of the fume hoodcontroller of the present invention, particularly illustrating theoperation of the feed forward control scheme, which is included in oneof the embodiments of the fume hood controller means;

FIG. 13 is a flow chart of a portion of the operation of the fume hoodcontroller means, particularly illustrating the operation of theproportional gain, integral gain and derivative gain control scheme;and,

FIG. 14 is a flow chart of a portion of the operation of the fume hoodcontroller means, particularly illustrating the operation of thecalibration of the feed forward control scheme.

DETAILED DESCRIPTION

It should be generally understood that a fume hood controller controlsthe flow of air through the fume hood in a manner whereby the effectivesize of the total opening to the fume hood, including the portion of theopening that is not covered by one or more sash doors will have arelatively constant average face velocity of air moving into the fumehood. This means that regardless of the area of the uncovered opening,an average volume of air per unit of surface area of the uncoveredportion will be moved into the fume hood. This protects the persons inthe laboratory from being exposed to noxious fumes or the like becauseair is always flowing into the fume hood, and out of the exhaust duct,and the flow is preferably controlled at a predetermined rate ofapproximately 75 to 125 cubic feet per minute per square feet ofeffective surface area of the uncovered opening. In other words, if thesash door or doors are moved to the maximum open position whereby anoperator has the maximum access to the inside of the fume hood forconducting experiments or the like, then the flow of air will mostlikely have to be increased to maintain the average face velocity at thepredetermined desired level.

Broadly stated, the present invention relates to a system whereby thepressure of a room containing one or more fume hoods, for example, suchas laboratory or the like is to maintained at a predetermined levelrelative to the pressure of a reference space in the building, which maybe the pressure in an adjacent corridor or an adjacent room or the like.It is often highly desirable to maintain the differential pressure in alaboratory room at a reduced level relative to the reference space, inorder to contain the noxious fumes so that they will not permeate beyondthe room. The system involves a room controller which is part of theheating, ventilating and air conditioning apparatus of the building. Theroom controller is of the type which can receive electrical signals fromeach of the fume hood controllers, which signals are proportional to thevolume of air that is being exhausted through each fume hood. Since eachfume hood can be exhausting an amount of air that can vary considerablydepending upon its initial setting of the desired average face velocityand the degree by which the sash doors are opened, it is veryadvantageous that the volume indicating signals be communicated fromeach of the fume hood controllers to the room controller so that it canmodulate the volume of air that is being supplied to the room whichassists it in maintaining the differential pressure at the desired levelwith relatively quick response times.

Turning now to the drawings, and particularly FIG. 1, a block diagram isshown of several fume hood controllers 20 interconnected with a roomcontroller 22, an exhaust controller 24 and a main control console 26.The fume hood controllers 20 are interconnected with the room controller22 and with the exhaust controller 24 and the main control console 26 ina local area network illustrated by line 28 which may be amulticonductor cable or the like. The room controller, the exhaustcontroller 24 and the main control console 26 are typically part of thebuilding main HVAC system in which the laboratory rooms containing thefume hoods are located. The fume hood controllers 20 are provided withpower through lines 30, which is at the proper voltage via a transformer32 or the like.

The room controller 22 preferably is of the type which is at leastcapable of providing a variable air volume to the room, and may be aLandis & Gyr Powers System 600 SCU controller. The room controller 22 iscapable of communicating over the LAN lines 28. When response time iscritical, the room controller preferably receives fume hood exhaust flowinformation from the fume hood controller as an analog signal directlythrough dedicated lines. This will bypass the LAN which may become busytransporting other information from the fume hood controllers to theroom controller. The room controller preferably is a System 600 SCUcontroller and is a commercially available controller for whichextensive documentation exists. The User Reference Manual, Part No.125-1753 for the System 600 SCU controller is specifically incorporatedby reference herein.

The room controller 22 receives signals via lines 81 from each of thefume hood controllers 20 that provides an analog input signal indicatingthe volume of air that is being exhausted by each of the fume hoodcontrollers 20 and a comparable signal from the exhaust flow sensor thatprovides an indication of the volume of air that is being exhaustedthrough the main exhaust system apart from the fume hood exhausts. Thesesignals coupled with signals that are supplied by a differentialpressure sensor 29 which indicates the pressure within the room relativeto the reference space enable the room controller to control the supplyof air that is necessary to maintain the differential pressure withinthe room at a slightly lower pressure than the reference space, i.e.,preferably within the range of about 0.01 to about 0.05 inches of water,which results in the desirable lower pressure of the room relative tothe reference space. However, it is not so low that it prevents personsinside the laboratory room from opening the doors to escape in the eventof an emergency, particularly if the doors open outwardly from the room.Also, in the event the doors open inwardly, the differential pressurewill not be so great that it will pull the door open due to excessiveforce being applied due to such pressure.

The differential pressure sensor 29 is preferably positioned in asuitable hole or opening in the wall between the room and the referencespace and measures the pressure on one side relative to the other.Alternatively, a velocity sensor may be provided which measures thevelocity of air moving through the opening which is directlyproportional to the pressure difference between the two spaces. Ofcourse, a lower differential pressure in the room relative to thereference space would mean that air would be moving into the room whichis also capable of being detected.

Referring to FIG. 2, a fume hood controller 20 is illustrated with itsinput and output connector ports being identified, and the fume hoodcontroller 20 is connected to an operator panel 34. It should beunderstood that each fume hood will have a fume hood controller 20 andthat an operator panel will be provided with each fume hood controller.The operator panel 34 is provided for each of the fume hoods and it isinterconnected with the fume hood controller 20 by a line 36 whichpreferably comprises a multi-conductor cable having eight conductors.The operator panel has a connector 38, such as a 6 wire RJ11 typetelephone jack, for example, into which a lap top personal computer orthe like may be connected for the purpose of inputting informationrelating to the configuration or operation of the fume hood duringinitial installation, or to change certain operating parameters ifnecessary. The operator panel 34 is preferably mounted to the fume hoodin a convenient location adapted to be easily observed by a person whois working with the fume hood.

The fume hood controller operator panel 34 includes a liquid crystaldisplay 40 which when selectively activated provides the visualindication of various aspects of the operation of the fume hood,including three digits 42 which provide the average face velocity. Thedisplay 40 illustrates other conditions such as low face velocity, highface velocity and emergency condition and an indication of controllerfailure. The operator panel may have an alarm 44, an emergency purgeswitch 46 which an operator can press to purge the fume hood in theevent of an accident. The operator panel has two auxiliary switches 48which can be used for various customer needs, including day/night modesof operation. It is contemplated that night time mode of operation wouldhave a different and preferably reduced average face velocity,presumably because no one would be working in the area and such a loweraverage face velocity would conserve energy. An alarm silence switch 50is also preferably provided to extinguish the alarm.

Fume hoods come in many different styles, sizes and configurations,including those which have a single sash door or a number of sash doors,with the sash doors being moveable vertically, horizontally or in bothdirection. Additionally, various fume hoods have different amounts ofby-pass flow, i.e., the amount of flow permitting opening that existseven when all of the sash doors are as completely closed as their designpermits. Other design considerations involve whether there is some kindof filtering means included in the fume hood for confining fumes withinthe hood during operation. While many of these design considerationsmust be taken into account in providing efficient and effective controlof the fume hoods, the apparatus can be configured to account forvirtually all of the above described design variables, and effective andextremely fast control of the fume hood ventilation is provided.

Referring to FIG. 3, there is shown a fume hood, indicated generally at60, which has a vertically operated sash door 62 which can be moved togain access to the fume hood and which can be moved to the substantiallyclosed position as shown. Fume hoods are generally designed so that evenwhen a door sash such as door sash 62 is completely closed, there isstill some amount of opening into the fume hood through which air canpass. This opening is generally referred to as the bypass area and itcan be determined so that its effect can be taken into consideration incontrolling the flow of air into the fume hood. Some types of fume hoodshave a bypass opening that is located above the door sash while othersare below the same. In some fume hoods, the first amount of movement ofa sash door will increase the opening at the bottom of the door shown inFIG. 3 for example, but as the door is raised, it will merely cut offthe bypass opening so that the effective size of the total opening ofthe fume hood is maintained relatively constant for perhaps the firstone-fourth amount of movement of the sash door 62 through its course oftravel.

Other types of fume hoods may include several horizontally moveable sashdoors 66 such as shown in FIGS. 4 and 5, with the doors being movable inupper and lower pairs of adjacent tracks 68. When the doors arepositioned as shown in FIGS. 4 and 5, the fume hood opening iscompletely closed and an operator may move the doors in the horizontaldirection to gain access to the fume hood. Both of the fumes hoods 60and 64 have an exhaust duct 70 which generally extends to an exhaustsystem which may be that of the HVAC apparatus previously described. Thefume hood 64 also includes a filtering structure shown diagrammaticallyat 72 which filtering structure is intended to keep noxious fumes andother contaminants from exiting the fume hood into the exhaust system.Referring to FIG. 6, there is shown a combination fume hood which hashorizontally movable doors 76 which are similar to the doors 66, withthe fume hood 74 having a frame structure 78 which carries the doors 76in suitable tracks and the frame structure 78 is also vertically movablein the opening of the fume hood.

The illustration of FIG. 6 has portions removed as shown by the breaklines 73 which is intended to illustrate that the height of the fumehood may be greater than is otherwise shown so that the frame structure78 may be raised sufficiently to permit adequate access to the interiorof the fume hood by a person. There is generally a by-pass area which isidentified as the vertical area 75, and there is typically a top lipportion 77 which may be approximately 2 inches wide. This dimension ispreferably defined so that its effect on the calculation of the openface area can be taken into consideration.

While not specifically illustrated, other combinations are alsopossible, including multiple sets of vertically moveable sash doorspositioned adjacent one another along the width of the fume hoodopening, with two or more sash doors being vertically moveable inadjacent tracks, much the same as residential casement windows.

The fume hood controller 20 is adapted to operate the fume hoods ofvarious sizes and configurations as has been described, and it is alsoadapted to be incorporated into a laboratory room where several fumehoods may be located and which may have exhaust ducts which merge into acommon exhaust manifold which may be a part of the building HVAC system.A fume hood may be a single self-contained installation and may have itsown separate exhaust duct. In the event that a single fume hood isinstalled, it is typical that such an installation would have a variablespeed motor driven blower associated with the exhaust duct whereby thespeed of the motor and blower can be variably controlled to therebyadjust the flow of air through the fume hood. Alternatively, and mosttypical for multiple fume hoods in a single area, the exhaust ducts ofeach fume hood are merged into one or more larger exhaust manifolds anda single large blower may be provided in the manifold system. In suchtypes of installations, control of each fume hood is achieved by meansof separate dampers located in the exhaust duct of each fume hood, sothat variation in the flow can be controlled by appropriatelypositioning the damper associated with each fume hood.

The fume hood controller is adapted to control virtually any of thevarious kinds and styles of fume hoods that are commercially available,and to this end, it has a number of input and output ports (lines,connectors or connections, all considered to be equivalent for thepurposes of the present description) that can be connected to varioussensors that may be used with the controller. As shown in FIG. 2, it hasdigital output or DO ports which interface with a digital signal/analogpressure transducer with an exhaust damper as previously described, butit also has an analog voltage output port for controlling a variablespeed fan drive if it is to be installed in that manner. There are fivesash position sensor ports for use in sensing the position of bothhorizontally and vertically moveable sashes and there is also an analoginput port provided for connection to an exhaust air flow sensor. Adigital input port for the emergency switch is provided and digitaloutput ports for outputting an alarm horn signal as well as an auxiliarysignal is provided.

As has been previously discussed and in accordance with the presentinvention, an analog voltage output port is also provided for providinga volume of flow signal to the room controller 22. This port isconnected to the room controller by individual lines 81 which extendfrom each of the fume hood controllers 22.

From the foregoing discussion, it should be appreciated that if thedesired average face velocity is desired to be maintained and the sashposition is changed, the size of the opening can be dramatically changedwhich may then require a dramatic change in the volume of air tomaintain the average face velocity. While it is known to control avariable air volume blower as a function of the sash position, the fumehood controller apparatus improves on that known method by incorporatingadditional control schemes which dramatically improve the capabilitiesof the control system in terms of maintaining relatively constantaverage face velocity in a manner whereby reactions to perturbations inthe system are quickly made.

To determine the position of the sash doors, a sash position sensor isprovided adjacent each movable sash door and it is generally illustratedin FIGS. 7, 8 and 9. Referring to FIG. 8, the door sash positionindicator comprises an elongated switch mechanism 80 of relativelysimple mechanical design which preferably consists of a relatively thinpolyester base layer 82 upon which is printed a strip of electricallyresistive ink 84 of a known constant resistance per unit length. Anotherpolyester base layer 86 is provided and it has a strip of electricallyconductive ink 88 printed on it. The two base layers 82 and 86 areadhesively bonded to one another by two beads of adhesive 90 located onopposite sides of the strip. The base layers are preferablyapproximately five-thousandths of an inch thick and the beads areapproximately two-thousandths of an inch thick, with the beads providinga spaced area between the conductive and resistive layers 88 and 84. Theswitching mechanism 80 is preferably applied to the fume hood by a layerof adhesive 92.

The polyester material is sufficiently flexible to enable one layer tobe moved toward the other in response to an actuator 94 carried by theappropriate door sash to which the strip is placed adjacent to so thatwhen the door sash is moved, the actuator 94 moves along the switchingmechanism 80 and provides contact between the resistive and conductivelayers which are then sensed by electrical circuitry to be describedwhich provides a voltage output that is indicative of the position ofthe actuator 94 along the length of the switching mechanism. Stated inother words, the actuator 94 is carried by the door and thereforeprovides an electrical voltage that is indicative of the position of thedoor sash.

The actuator 94 is preferably spring biased toward the switchingmechanism 80 so that as the door is moved, sufficient pressure isapplied to the switching mechanism to bring the two base layers togetherso that the resistive and conductive layers make electrical contact withone another and if this is done, the voltage level is provided. Byhaving the switching mechanism 80 of sufficient length so that the fullextent of the travel of the sash door is provided as shown in FIG. 3,then an accurate determination of the sash position can be made. Itshould be understood that the illustration of the switching mechanism 80in FIGS. 3 and 5 is intended to be diagrammatic, in that the switchingmechanism is preferably actually located within the sash frame itselfand accordingly would not be visible as shown. The width and thicknessdimensions of the switching mechanism are so small that interferencewith the operation of the sash door is virtually no problem. Theactuator 94 can also be placed in a small whole that may be drilled inthe door or it may be attached externally at one end thereof so that itcan be in position to operate the switch 80. In the vertical moveablesash doors shown in FIGS. 3 and 6, a switching mechanism 80 ispreferably provided in one or the other of the sides of the sash frame,whereas in the fume hoods having horizontally movable doors, it ispreferred that the switching mechanism 80 be placed in the top of thetracks 68 so that the weight of the movable doors do not operate theswitching mechanism 80 or otherwise damage the same. It is alsopreferred that the actuator 94 is located at one end of each of thedoors for reasons that are described in the cross-referenced applicationentitled Apparatus for determining the position of a moveable structurealong a track, by Egbers et al., Ser. No. 52496.

Turning to FIG. 9, the preferred electrical circuitry which generatesthe position indicating voltage is illustrated, and this circuitry isadapted to provide two separate voltages indicating the position of twodoor sashes in a single track. With respect to the cross-section shownin FIG. 5, there are two horizontal tracks, each of which carries twodoor sashes and a switching mechanism 80 is provided for each of thetracks as is a circuit as shown in FIG. 9, thereby providing a distinctvoltage for each of the four sash doors as shown.

The switching mechanism is preferably applied to the fume hood with alayer of adhesive 92 and the actuator 94 is adapted to bear upon theswitching mechanism at locations along the length thereof. Referring toFIG. 7, a diagrammatic illustration of a pair of switching mechanisms isillustrated such as may occur with respect to the two tracks shown inFIG. 5. A switching mechanism 80 is provided with each track and thefour arrows illustrated represent the point of contact created by theactuators 94 which result in a signal being applied on each of the endsof each switching mechanism, with the magnitude of the signalrepresenting a voltage that is proportional to the distance between theend and the nearest arrow. Thus, a single switching mechanism 80 isadapted to provide position indicating signals for two doors located ineach track. The circuitry that is used to accomplish the voltagegeneration is shown in FIG. 9 and includes one of these circuits foreach track. The resistive element is shown at 84 and the conductiveelement 88 is also illustrated being connected to ground with two arrowsbeing illustrated, and represented the point of contact between theresistive and conductive elements caused by each of the actuators 94associated with the two separate doors. The circuitry includes anoperational amplifier 100 which has its output connected to the base ofa PNP transistor 102, the emitter of which is connected to a source ofpositive voltage through resistor 104 into the negative input of theoperational amplifier, the positive input of which is also connected toa source of positive voltage of preferably approximately five volts. Thecollector of the transistor 102 is connected to one end of the resistiveelement 84 and has an output line 106 on which the voltage is producedthat is indicative of the position of the door.

The circuit operates to provide a constant current directed into theresistive element 84 and this current results in a voltage on line 106that is proportional to the resistance value between the collector andground which changes as the nearest point of contact along theresistance changes. The operational amplifier operates to attempt todrive the negative input to equal the voltage level on the positiveinput and this results in the current applied at the output of theoperational amplifier varying in direct proportion to the effectivelength of the resistance strip 84. The lower portion of the circuitryoperates the same way as that which has been described and it similarlyproduces a voltage on an output line 108 that is proportional to thedistance between the connected end of the resistance element 84 and thepoint of contact that is made by the actuator 94 associated with theother sash door in the track.

Referring to the composite electrical schematic diagram of the circuitryof the fume hood controller, if the separate drawings FIGS. 10a, 10b,10c, 10d and 10e are placed adjacent one another in the manner shown inFIG. 10, the total electrical schematic diagram of the fume hoodcontroller 20 is illustrated. The operation of the circuitry of FIGS.10a through 10e will not be described in detail. The circuitry is drivenby a microprocessor and the important algorithms that carry out thecontrol functions of the controller will be hereinafter described.Referring to FIG. 10c, the circuitry includes a Motorola MC 68HC11microprocessor 120 which is clocked at 8 MHz by a crystal 122. Themicroprocessor 120 has a databus 124 that is connected to a tri-statebuffer 126 (FIG. 10d) which in turn is connected to an electricallyprogrammable read only memory 128 that is also connected to the databus124. The EPROM 128 has address lines A0 through A7 connected to thetri-state buffer 126 and also has address lines A8 through A14 connectedto the microprocessor 120.

The circuitry includes a 3 to 8-bit multiplexer 130, a data latch 132(see FIG. 10d), a digital-to-analog converter 134, which is adapted toprovide the analog outputs indicative of the volume of air beingexhausted by the fume hood, which information is provided to roomcontroller 22 as has been previously described with respect to FIG. 2.Referring to FIG. 10b, an RS232 driver 136 is provided for transmittingand receiving information through the hand held terminal. The circuitryillustrated in FIG. 9 is also shown in the overall schematic diagramsand is in FIGS. 10a and 10b. The other components are well known andtherefore need not be otherwise described.

As previously mentioned, the fume hood control apparatus utilizes a flowsensor preferably located in the exhaust duct 70 to measure the airvolume that is being drawn through the fume hood. The volume flow ratemay be calculated by measuring the differential pressure across amulti-point pitot tube or the like. The preferred embodiment utilizes adifferential pressure sensor for measuring the flow through the exhaustduct and the fume hood control apparatus utilizes control schemes toeither maintain the flow through the hood at a predetermined averageface velocity, or at a minimum velocity in the event the fume hood isclosed or has a very small bypass area.

The fume hood controller can be configured for almost all known types offume hoods, including fume hoods having horizontally movable sash doors,vertically movable sash doors or a combination of the two. As can beseen from the illustrations of FIGS. 2 and 10, the fume hood controlleris adapted to control an exhaust damper or a variable speed fan drive,the controller being adapted to output signals that are compatible witheither type of control. The controller is also adapted to receiveinformation defining the physical and operating characteristics of thefume hood and other initializing information. This can be input into thefume hood controller by means of the hand held terminal which ispreferably a lap top computer that can be connected to the operatorpanel 34. The information that should be provided to the controllerinclude the following, and the dimensions for the information are alsoshown. It should be appreciated that the day/night operation may beprovided, but is not the preferred embodiment of the system; if it isprovided, the information relating to such day/night operation should beincluded.

Operational information:

1. Time of day;

2. Set day and night values for the average face velocity (SVEL), feetper minute or meters per second;

3. Set day and night values for the minimum flow, (MINFLO), in cubicfeet per minute;

4. Set day and night values for high velocity limit (HVEL), F/m orM/sec;

5. Set day and night values for low velocity limit (LVEL), F/m or M/sec;

6. Set day and night values for intermediate high velocity limit (MVEL),F/m or M/sec;

7. Set day and night values for intermediate low velocity limit (IVEL),F/m or M/sec;

8. Set the proportional gain factor (KP), analog output per error inpercent;

9. Set the integral gain factor (KI), analog output multiplied by timein minutes per error in percent;

10. Set derivative gain factor (KD), analog output multiplied by time inminutes per error in percent;

11. Set feed forward gain factor (KF) if a variable speed drive is usedas the control equipment instead of a damper, analog output per CFM;

12. Set time in seconds (DELTIME) the user prefers to have the fullexhaust flow in case the emergency button is activated;

13. Set a preset percent of last exhaust flow (SAFLOQ) the user wishesto have once the emergency switch is activated and DELTIME is expired.

The above information is used to control the mode of operation and tocontrol the limits of flow during the day or night modes of operation.The controller includes programmed instructions to calculate the stepsin paragraphs 3 through 7 in the event such information is not providedby the user. To this end, once the day and night values for the averageface velocity are set, the controller 20 will calculate high velocitylimit at 120% of the average face velocity, the low velocity limit at80% and the intermediate limit at 90%. It should be understood thatthese percentage values may be adjusted, as desired. Other informationthat should be input include the following information which relates tothe physical construction of the fume hood. It should be understood thatsome of the information may not be required for only vertically orhorizontally moveable sash doors, but all of the information may berequired for a combination of the same:

14. Input the number of vertical segments;

15. Input the height of each segment, in inches;

16. Input the width of each segment, in inches;

17. Input the number of tracks per segment;

18. Input the number of horizontal sashes per track;

19. Input the maximum sash height, in inches;

20. Input the sash width, in inches;

21. Input the location of the sash sensor from left edge of sash, ininches;

22. Input the by-pass area per segment, in square inches;

23. Input the minimum face area per segment, in square inches;

24. Input the top lip height above the horizontal sash, in inches;

The fume hood controller 20 is programmed to control the flow of airthrough the fume hood by carrying out a series of instructions, anoverview of which is contained in the flow chart of FIG. 11. Afterstart-up and outputting information to the display and determining thetime of day, the controller 20 reads the initial sash positions of alldoors (block 150), and this information is then used to compute the openface area (block 152). If not previously done, the operator can set theaverage face velocity set point (block 154) and this information is thenused together with the open face area to compute the exhaust flow setpoint (SFLOW) (block 156) that is necessary to provide the predeterminedaverage face velocity given the open area of the fume hood that has beenpreviously measured and calculated. The computed fume hood exhaust setpoint is then compared (block 158) with a preset or required minimumflow, and if computed set point is less than the minimum flow, thecontroller sets the set point flow at the preset minimum flow (block160). If it is more than the minimum flow, then it is retained (block162) and it is provided to both of the control loops.

If there is a variable speed fan drive for the fume controller, i.e.,several fume hoods are not connected to a common exhaust duct andcontrolled by a damper, then the controller will run a feed-forwardcontrol loop (block 164) which provides a control signal that is sent toa summing junction 166 which control signal represents an open loop typeof control action. In this control action, a predicted value of thespeed of the blower is generated based upon the calculated opening ofthe fume hood, and the average face velocity set point. The predictedvalue of the speed of the blower generated will cause the blower motorto rapidly change speed to maintain the average face velocity. It shouldbe understood that the feed forward aspect of the control is onlyinvoked when the sash position has been changed and after it has beenchanged, then a second control loop performs the dominant control actionfor maintaining the average face velocity constant in the event that avariable speed blower is used to control the volume of air through thefume hood.

After the sash position has been changed and the feed forward loop hasestablished the new air volume, then the control loop switches to aproportional integral derivative control loop and this is accomplishedby the set flow signal being provided to block 168 which indicates thatthe controller computes the error by determining the absolute value ofthe difference between the set flow signal and the flow signal asmeasured by the exhaust air flow sensor in the exhaust duct. Any errorthat is computed is applied to the control loop identified as theproportional-integral-derivative control loop (PID) to determine anerror signal (block 170) and this error signal is compared with theprior error signal from the previous sample to determine if that erroris less than a deadband error (block 172). If it is, then the priorerror signal is maintained as shown by block 174, but if it is not, thenthe new error signal is provided to output mode 176 and it is applied tothe summing junction 166. That summed error is also compared with thelast output signal and a determination is made if this is within adeadband range (block 180) which, if it is, results in the last orprevious output being retained (block 182). If it is outside of thedeadband, then a new output signal is provided to the damper control orthe blower (block 184).

In the event that the last output is the output as shown in block 182,the controller then reads the measured flow (MFLOW) (block 186) and thesash positions are then read (block 188) and the net open face area isrecomputed (block 190) and a determination made as to whether the newcomputed area less the old computed area is less than a deadband (block192) and if it is, then the old area is maintained (block 194) and theerror is then computed again (block 168). If the new area less the oldarea is not within the deadband, then the controller computes a newexhaust flow set point as shown in block 156.

One of the significant advantages of the fume hood controller is that itis adapted to execute the control scheme in a repetitive and extremelyrapid manner. The exhaust sensor provides flow signal information thatis inputted to the microprocessor at a speed of approximately one sampleper 100 milliseconds and the control action described in connection withFIG. 11 is completed approximately every 100 milliseconds. The sash doorposition signals are sampled by the microprocessor every 200milliseconds. The result of such rapid repetitive sampling and executingof the control actions results in extremely rapid operation of thecontroller. It has been found that movement of the sash will result inadjustment of the air flow so that the average face velocity is achievedwithin a time period of only approximately 3-4 seconds after the sashdoor reposition has been stopped. This represents a dramatic improvementover existing fume hood controllers.

In the event that the feed forward control loop is utilized, thesequence of instructions that are carried out to accomplish running ofthis loop is shown in the flow chart of FIG. 12, which has thecontroller using the exhaust flow set point (SFLOW) to compute thecontrol output to a fan drive (block 200), which is identified as signalAO that is computed as an intercept point plus the set flow multipliedby a slope value. The intercept is the value which is a fixed outputvoltage to a fan drive and the slope in the equation correlates exhaustflow rate and output voltage to the fan drive. The controller then readsthe duct velocity (DV) (block 202), takes the last duct velocity sample(block 204) and equates that as the duct velocity value and starts thetiming of the maximum and minimum delay times (block 206) which thecontroller uses to insure whether the duct velocity has reached steadystate or not. The controller determines whether the maximum delay timehas expired (block 208), and if it has, provides the output signal atoutput 210. If the max delay has not expired, the controller determinesif the absolute value of the difference between the last duct velocitysample and the current duct velocity sample is less than or equal to adead band value (block 212). If it is not less than the dead band value,the controller then sets the last duct value as equal to the presentduct value sample (block 214) and the controller then restarts theminimum delay timing function (block 216). Once this is accomplished,the controller again determines whether the max delay has expired (block208). If the absolute value of the difference between the last ductvelocity and the present duct velocity sample is less than the deadband, the controller determines whether the minimum delay time hasexpired which, if it has as shown from block 218, the output is providedat 210. If it has not, then it determines if the max delay has expired.

Turning to the proportional-integral-derivative or PID control loop, thecontroller runs the PID loop by carrying out the instructions shown inthe flow chart of FIG. 13. The controller uses the error that iscomputed by block 168 (see FIG. 11) in three separate paths. Withrespect to the upper path, the controller uses the preselectedproportional gain factor (block 220) and that proportional gain factoris used together with the error to calculate the proportional gain(block 222) and the proportional gain is output to a summing junction224.

The controller also uses the error signal and calculates an integralterm (block 226) with the integral term being equal to the priorintegral sum (ISUM) plus the product of loop time and any error and thiscalculation is compared to limits to provide limits on the term. Theterm is then used together with the previously defined integral gainconstant block 230) and the controller than calculates the integral gain(block 232) which is the integral gain constant multiplied by theintegration sum term. The output is then applied to the summing junction224.

The input error is also used by the controller to calculate a derivativegain factor which is done by the controller using the previously definedderivative gain factor from block 234 which is used together with theerror to calculate the derivative gain (block 236) which is thereciprocal of the time in which it is required to execute the PID loopmultiplied by the derivative gain factor multiplied by the currentsample error minus the previous sample error with this result beingprovided to the summing junction 224.

The control action performed by the controller 20 as illustrated in FIG.13 provides three separate gain factors which provide steady statecorrection of the air flow through the fume hood in a very fast actingmanner. The formation of the output signal from the PID control looptakes into consideration not only the magnitude of the error, but as aresult of the derivative gain segment of control, the rate of change ofthe error is considered and the change in the value of the gain isproportional to the rate of change. Thus, the derivative gain can seehow fast the actual condition is changing and works as an "anticipator"in order to minimize error between the actual and desired condition. Theintegral gain develops a correction signal that is a function of theerror integrated over a period of time, and therefore provides anynecessary correction on a continuous basis to bring the actual conditionto the desired condition. The proper combinations of proportional,integral and derivative gains will make the loop faster and reach thedesired conditions without any overshoot.

A significant advantage of the PID control action is that it willcompensate for perturbations that may be experienced in the laboratoryin which the fume hood may be located in a manner in which othercontrollers do not. A common occurrence in laboratory rooms which have anumber of fume hoods that are connected to a common exhaust manifold,involves the change in the pressure in a fume hood exhaust duct that wascaused by the sash doors being moved in another of the fume hoods thatis connected to the common exhaust manifold. Such pressure variationswill affect the average face velocity of those fume hoods which had nochange in their sash doors. However, the PID control action may adjustthe air flow if the exhaust duct sensor determines a change in thepressure. To a lesser degree, there may be pressure variations producedin the laboratory caused by opening of doors to the laboratory itself,particularly if the differential pressure of the laboratory room ismaintained at a lesser pressure than a reference space such as thecorridor outside the room, for example.

It is necessary to calibrate the feed forward control loop and to thisend, the instructions illustrated in the flow chart of FIG. 14 arecarried out. When the initial calibration is accomplished, it ispreferably done through the hand held terminal that may be connected tothe operator panel via connector 38, for example. The controller thendetermines if the feed forward calibration is on (block 242) and if itis, then the controller sets the analog output of the fan drive to avalue of 20 percent of the maximum value, which is identified as valueA01 (block 244). The controller then sets the last sample duct velocity(LSDV) as the current duct velocity (CDV) (block 246) and starts themaximum and minimum timers (block 248). The controller ensures thesteady state duct velocity in the following way. First by checkingwhether the max timer has expired, and then, if the max timer has notexpired, the controller determines if the absolute value of the lastsample duct velocity minus the current duct velocity is less than orequal to a dead band (block 270), and if it is, the controllerdetermines if the min timer has expired (block 272). If it has not, thecontroller reads the current duct velocity (block 274). If the absolutevalue of the last sample duct velocity minus the current duct velocityis not less than or equal to a dead band (block 270), then the lastsample duct velocity is set as the current duct velocity (block 276) andthe mintimer is restarted (block 278) and the current duct velocity isagain read (block 274). In case either the max timer or min timer hasexpired, the controller then checks the last analog output value to thefan drive (252) and inquires whether the last analog output value was 70percent of the maximum output value (block 254). If it is not, then itsets the analog output value to the fan drive at 70 percent of the maxvalue A02 (block 256) and the steady state duct velocity correspondingto A01. The controller then repeats the procedure of ensuring steadystate duct velocity when analog output is A02 (block 258). If it is atthe 70 percent of max value, then the duct velocity corresponds tosteady state velocity of A02 (block 258). Finally, the controller (block262) calculates the slope and intercept values.

The result of the calibration process is to determine the duct flow at20% and at 70% of the analog output values, and the measured flowenables the slope and intercept values to be determined so that the feedforward control action will accurately predict the necessary fan speedwhen sash door positions are changed.

From the foregoing detailed description, it should be appreciated thatan improved system has been described which has advantages over theprior art in terms of effectively maintaining a desired differentialpressure in a room where a plurality of fume hoods are present.

While various embodiments of the present invention have been shown anddescribed, it should be understood that various alternatives,substitutions and equivalents can be used, and the present inventionshould only be limited by the claims and equivalents thereof.

Various features of the present invention are set forth in the followingclaims.

What is claimed is:
 1. A system for controlling the differentialpressure at a predetermined level between a room such as a laboratory orthe like and a reference space such as a corridor or the like, both ofwhich are located in a building having a building heating, ventilatingand air conditioning apparatus, and in which room at least one fume hoodis located, each fume hood being of the type which has at least onemoveable sash door adapted to at least partially cover the opening asthe fume hood sash door is moved, each fume hood having an exhaust ductthat is in communication with an exhaust apparatus for expelling air andfumes from the room, each fume hood having a means for measuring theactual flow of air through its associated exhaust duct and generating anactual flow signal that is indicative of the actual flow of air throughthe exhaust duct, each fume hood being controlled by a fume hoodcontroller means for controlling a flow modulating means associated witheach fume hood and its associated exhaust duct to maintain a desiredface velocity through the uncovered portion of the opening, said systemcomprising:room controlling means for controlling at least the volume ofair that is supplied to the room from the heating and air conditioningapparatus of the building; means for interconnecting each of said fumehood controller means to said room controlling means so that the signalsrepresenting the actual flow from each of the fume hood controllers iscommunicated to said room controlling means, said room controlling meansbeing adapted to receive and sum the communicated signals from each ofsaid fume hood controller means and thereby determine the volume of airbeing exhausted from the room by the fume hoods, said room controllingmeans utilizing said adjusting means to vary the volume of air that issupplied to the room to replace the volume of air being exhausted fromthe room at a rate necessary to maintain the differential pressure atsaid predetermined level.
 2. Apparatus as defined in claim 1 whereinsaid interconnecting means comprises conducting means extending fromeach fume hood controller means to said room controlling means. 3.Apparatus as defined in claim 1 wherein each of said fume hoodcontroller means is adapted to transmit a voltage level that isproportional to the actual flow of air through the exhaust duct of thefume hood to which the fume hood controller means is connected. 4.Apparatus as defined in claim 1 wherein said actual flow measuring meansis positioned to measure the air flow in the exhaust duct of the fumehood.
 5. Apparatus as defined in claim 1 wherein a plurality of fumehoods are located in the room.
 6. Apparatus for controlling thedifferential pressure at a predetermined level within a room in abuilding having a building heating, ventilating and air conditioning andexhaust system with reference to the pressure in another space in thebuilding such as a corridor or the like, and in which room at least onefume hood is located, each fume hood being of the type which has atleast one moveable sash door adapted to at least partially cover theopening as the fume hood sash door is moved, each fume hood having anexhaust duct that is in communication with the exhaust system forexpelling air and fumes from the fume hoods, each fume hood having ameans for measuring the actual flow of air through the exhaust duct andgenerating an actual flow signal that is indicative of the actual flowof air through the exhaust duct, each fume hood being controlled by afume hood controller means for controlling a flow modulating meansassociated with each fume hood and its associated exhaust duct tomaintain a desired face velocity through the uncovered portion of theopening, said apparatus comprising:room controlling means forcontrolling at least the volume of air that is supplied to the room bythe building heating and air conditioning system; means forinterconnecting each of said fume hood controller means to said roomcontrolling means so that the actual flow signals from each of the fumehood controllers is communicated to said room controlling means, saidroom controlling means being adapted to receive and sum the actual flowsignals from each of said fume hood controller means and therebydetermine the volume of air being exhausted from the room through thefume hood exhaust ducts, said room controlling means adjusting thevolume of air that is supplied to the room to replace the volume of airbeing exhausted form the room at a rate sufficient to maintain thedifferential pressure at said predetermined level.
 7. Apparatus asdefined in claim 5 wherein a plurality of fume hoods are located in theroom.
 8. A system for controlling the differential pressure between aroom such as a laboratory or the like and a reference space, in abuilding having a building temperature control apparatus, and in whichroom at least one fume hood is located, each fume hood being of the typewhich has at least one moveable sash door adapted to at least partiallycover the opening as the fume hood sash door is moved, each fume hoodhaving an exhaust duct that is in communication with an exhaustapparatus for expelling air and fumes from the room, each fume hoodhaving a means for measuring the actual flow of air through itsassociated exhaust duct and generating a signal that is representativeof the actual flow of air through the associated exhaust duct, each fumehood being controlled by a fume hood controller means for controlling aflow modulating means associated with each fume hood and its associatedexhaust duct to maintain a desired face velocity through the uncoveredportion of the opening, said system comprising:room controlling meansfor controlling at least the volume of air that is supplied to the roomfrom the heating and air conditioning apparatus of the building; meansfor interconnecting each of said fume hood controller means to said roomcontrolling means so that the signals representing the actual flow fromeach of the fume hood controllers is communicated to said roomcontrolling means, said room controlling means being adapted to receiveand sum the communicated signals from each of said fume hood controllermeans and thereby determine the volume of air being exhausted from theroom by the fume hoods, said room controlling means utilizing saidadjusting means to vary the volume of air that is supplied to the roomto replace the volume of air being exhausted form the room at a ratenecessary to maintain the desired differential pressure.